Treat a null elementNs in getPropertyType/geAtributeType as the HTML …

[fred-wang]

…namespace

This is the behavior that Chromium and WebKit currently implement, see https://phabricator.services.mozilla.com/D226547

Currently, it seems that the "interface for localName and elementNs" would fallback to Element, similarly to what happens with document.createElementNS(null, "div") for example.

Note that a null attrNs is already handled by the spec.

See also https://github.com/w3c/trusted-types/issues/553 for details.

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[fred-wang]

cc @lukewarlow @koto